Firstly click on start and type in the search box troubleshooting and click on it scroll down to windows updates click on it and then repair updates . Allow this to happen and see what it repairs. Then try to install SP1 if it still doesn't install then head over to the Microsoft Download site and download and run the Update Readiness Tool which will correct any problems this tool should only be used as a last resource . Generally the trouble shooter will fix it. Make sure that all of your updates are installed prior to deploying the service pack.